This invention related to improved control means for regulated switching power supplies, particularly power supplies of the single-ended type. Specifically, the invention relates to an efficient regulating control circuit providing several functions for both feed-forward line regulation, and output regulation under varying load conditions.
Switching power supplies are among the most widely used types of power supplies in the electronics industry. The are used extensively in digital computers and other data processing applications, electronic test systems, avionics equipment and, in general, throughout all branches of industry. Because of their universal application, and since often several regulated power supplies are required in a single electronic installation, cost is a critical factor of choice.
Of equal importance is the ability of the power supply to satisfy stringent technical requirements. With the increasing sophistication of electronic systems, together with the trend toward electronic miniaturization and extremely high electronic density, operating specifications have been made stricter. Obviously, a power supply must accurately control the regulated output parameters within specified limits. Additionally, it must have the capability of protecting itself against fluctuations in line voltage and inadvertent overloading by the user. Thus the power supply desirably should be able to protect itself against heavy current overloads, even short circuits, and should be able to withstand extended periods of brownout (i.e., low input power caused by reductions in line voltage) without damage.
With the high electronic density found in modern day digital computers, control of heat dissipation within the power supply itself is critical. In order to be compatible with environments of high electronic and power density, the power supply should have a small physical size and consume small amounts of power for control functions. If the power supply is small, it is essential to control the power that develops within the power supply during operation; otherwise, heat will accumulate within the power supply chassis. If such generated heat is not minimized, large heat sinking masses must be incorporated in order to transfer heat to points external to the chassis. Such heat sinking masses raise the cost of the power supply significantly and add to its weight and size.
As is well known in the art, switching power supplies incorporate controllable switch means which provide current pulses to an output circuit that converts the current pulses into a direct current output. Regulation of the output parameter, e.g., output voltage, is gained by controlling the duty cycle of switch operation. Control of the switched power duty cycle is obtained by continuously and automatically adjusting the duration of switch activation in accordance with incremental variations in the output voltage.
In a balanced type of regulated switching power supply, an inverter customarily is used to convert unregulated direct current (derived from the external source) into an alternating current signal. This a.c. signal controls alternately conducting switching transistors for transferring power to an output transformer having suitable filters to convert the switched power back to direct current power at the output. A disadvantage of the balanced type of power supply is its susceptibility to transformer saturation due to volt-time unbalances. The characteristics of the switched current must be carefully controlled in order to avoid high d.c. components in the output transformer. Perhaps its most significant disadvantage is cost. Because two power switching transistors and a larger center-tapped output transformer are required, the balanced power supply is more costly to manufacture than single-ended supplies using a single switching transistor and no center transformer tap.
In the single-ended type of switching power supply, output regulation is achieved in essentially the same way, i.e., the switched current duty cycle is controlled in order to maintain the output parameter (voltage and/or current) at the desired level. This output voltage is a function of the input line voltage, the tranformer winding turns ratio, and the duty cycle of the current switched to the output transformer. Nevertheless, here is a limit to which the duty cycle can be extended in order to compensate, for example, for excessively low input line voltages. This is because the volt-time products at the transformer primary must be equal for switch conduction and non-conduction periods.
For any given input voltage, the volt-time product increases with increasing switch conduction time. Thus, as switch conduction time increases, the voltage generated in the primary of the transformer during the non-conducting portion of the cycle will also increase. Theoretically, if the switch were allowed to conduct for nearly the entire switching cycle, the non-conduction period would become infinitestimally small and, accordingly, the voltage developed across the switch would become infinitely large. Indeed, this voltage would greatly exceed the limits of the switching transistor and it would quickly burn out.
It is thus necessary in single-ended switching power supplies, to carefully limit the duty cycle of the switching transistor to a maximum value selected to preclude the primary transformer voltage from exceeding the switch's breakdown tolerance. Many power-line driven single-ended power supplies are designed to limit this duty cycle to about 50% and, thus, the switch conducts for not more than about one-half of the available switching cycle. This limitation restricts the degree of attainable output regulation as a function of load or input line variations. Variations in output calling for a duty cycle greater than 50% will result in loss of regulation. In the present invention, the duty cycle may be extended to 70%-80%, thus permitting regulation over wider swings in load current and line voltage.
Another important requirement for regulated power supplies used in data processing applications is that a minimum "hold-up" time be achieved for power failure. Thus, the power supply normally must be able to maintain a minimum output voltage under full rated load for short periods of time sufficient to permit the computer memories to be switched to a back-up battery or auxiliary power. This requires a sufficiently large input filter capacitance to store the necessary energy after input power is lost. If regulation can be achieved, as in the present invention, over a broader range of input voltages, then a smaller capacitor can be used. On the other hand, if, under power "brownout" (i.e., very low line voltage) conditions, adequate control measures are not implemented, then the period of switch conduction can grow long enough to drive the output transformer into saturation with a resultant loss in output voltage. Moreover, since maximum heat dissipation in the switching transistor occurs during brownout, the switching device can exceed its thermal limitations and be destroyed. The present invention incorporates means to avoid these operational restrictions.
Over-current protection is another requirement for regulated power supplies. In order to protect the power supply against burn out, as well as to protect the user's equipment against damage due to abnormal current demands, as where there is an inadvertent short circuit, means must be provided to limit the power supply current output. The present invention incorporates current limiting in a manner unique to switching power supplies.